Suitable doped polycrystalline silicon structures have been used, for example, as the gate material in insulated gate field effect transistor (IGFET) devices as well as in other similar structures. It is known that improved lateral conductivity can be achieved by replacing doped polycrystalline silicon with a two-layer structure wherein a layer of a refractory metal silicide overlies a layer of doped polycrystalline silicon. See, for example, U.S. Pat. Nos. 4,285,761 and 4,141,022. The two-layer structure is utilized because of its greatly increased speed in comparison with doped polysilicon.
The fabrication of two-layer systems wherein a refractory metal silicide overlies doped polycrystalline silicon has been beset with a number of technical problems. Because of the demand for ever-higher speeds of operation, however, there has been considerable effort in the electronics industry to resolve these problems. In spite of this effort, some serious problems still remain. For example, previous methods of etching such structures are isotropic and will cause undercutting of both layers. In addition, such methods have poor selectivity in that they will continue to etch the layer of silicon dioxide which conventionally underlies such structures. For example, carbon tetrafluoride/oxygen, which has been used to etch such structures, will cause severe undercutting, particularly of the polycrystalline silicon layer when used at high pressures and will etch the underlying oxide when used at low pressures. This mixture is also characterized by poor line-width control, regardless of the operating pressure.
Another problem associated with the subject structures is that, frequently, the refractory metal silicide layer will delaminate from the polycrystalline silicon layer during annealing. In addition, the etching of such structures has been characterized by poor reproducibility, regardless of the etchant mixture utilized.
In accordance with this invention, two-layer structures of doped polycrystalline silicon and refractory metal silicide are anisotropically etched by a highly reproducible method which suffers from none of these disadvantages.